1. Field of the Invention
Embodiments of the invention relate to a pressure relief valve system. Embodiments of the invention relate to an internal pressure relief valve for a valve actuator. Embodiments of the invention relate to an internal pressure relieving hydraulic actuator for use with a valve, such as a gate valve.
2. Description of the Related Art
Various designs of actuators exist that operate to open and close valves in a variety of uses. The petroleum industry, for example, utilizes these actuators to operate gate valves. There are numerous types of gate valves, including direct acting gate valve and reverse acting gate valves. A gate valve may generally comprise a valve body having a central axis aligned with inlet and outlet passages, and a space between the inlet and outlet passages in which a slide, or gate, may be moved perpendicular to the central axis to open and close the valve and thus selectively block fluid flow through tubing. Positioning gate valves along tubing at various locations provides the ability to control and direct the flow of fluids through the tubing.
One type of actuator that is used to open and close a gate valve includes a dual action hydraulic actuator, which is typically mounted on top of the gate valve. This type of actuator has a piston that can be pressurized on a first side to impart an axial force in a first direction and can be pressurized on an opposite side to impart an axial force in an opposite direction. This type of actuator is therefore referred to as a dual action or “double acting” hydraulic actuator. A shaft may be connected to the actuator piston and used to impart the axial force to the gate valve. The shaft that may be connected to a valve stem extending from the valve body of a gate valve, and the valve stem is in turn connected to a gate located within the valve body. The shaft is used to impart axial movement to the valve stem to move the gate between an open position and a closed position.
In operation, when it is desired to open or close the gate valve (depending on the type of valve), a hydraulic fluid may be supplied to a first side of the actuator piston to impart an axial force, such as a downward or push force, to the valve stem of the gate valve via the actuator shaft. When it is desired to close or open the gate valve (depending on the type of valve), the hydraulic fluid may be supplied to the opposite side of the actuator piston to impart an opposite axial force, such as an upward or pull force, to the valve stem of the gate valve via the actuator shaft. The gate is generally connected to the end of the valve stem so that the downward and upward travel of the valve stem will situate the gate in the open and closed positions, respectively.
The use of a hydraulic actuator with a gate valve provides many benefits, but also encounters a few problems. One problem arises when an excessive amount of pressure is inadvertently supplied to the piston to ensure that the gate valve is fully open or closed. This excessive amount of pressure imparts an excessive force to the actuator shaft and thus the valve stem, which can cause damage to either member, as well as the gate and valve body. For example, the valve stem includes an outer shoulder that is configured to engage an inner shoulder of the valve body when the gate valve is in a closed position. This shoulder engagement provides a metal-to-metal seal between the valve stem and the valve body when the gate valve is in the closed position. This shoulder engagement may also prevent removal of the valve stem from the valve body. Continued pressurization of the actuator piston after engagement may cause excessive compressive forces in the metal-to-metal seal, thereby damaging the seal, and/or may actually shear the valve stem through the shoulder of the valve body, thereby damaging both the valve stem and body.
Pressure relief valves have been developed for relieving excessive pressures applied to the types of actuators described above. These pressure relief valves, however, are typically mounted external to the actuator, which make the actuators bulky, awkward, and difficult to use in compact spaces. Further, these exposed pressure relief valves are susceptible to being broken off of the actuator and to being tampered with. Further still, because of the many fluid lines required when external pressure relief valves are connected to an actuator, the relief valve is more susceptible to mechanical failure due to the mounting of the actuator, the rough conditions of usage, and the limited space where these actuators may be installed.
Therefore, there is a need for an improved pressure relief system for use with an actuator for a valve.